The present invention relates to a PSA (Pressure Swing Adsorption) method for separating a gas mixture containing hydrogen (H2) and hydrocarbon type (CnHm) impurities, in which the gas mixture to be purified is contacted with an activated charcoal and silica gel in order to adsorb the impurities present in the gas mixture to be treated, and to produce a hydrogen-rich stream, and further to produce a waste gas stream at a regeneration pressure between 2 and 10 bar absolute.
The PSA method is very frequently used for separating and for purifying gases.
In the case of the treatment of hydrogen-rich gases, this method is suitable for generating a pure hydrogen stream, typically with purity above 99% by volume, and a hydrogen-poor waste gas concentrating the other species present in the initial gas mixture to be purified.
Any PSA method is characterized by two main steps, as follows:                an adsorption phase in which the feed gas is contacted with one or more beds each containing one or more adsorbents, at an adsorption pressure (P) at which the compounds other than hydrogen are adsorbed and hence retained on the solid adsorbent(s). The gas leaving the bed is purified hydrogen which is produced at a production pressure (P′) close to the adsorption pressure (P); generally, the difference between these pressures P and P′ is less than 1 bar.        a desorption phase in which the adsorbent(s) is(are) scavenged by an elution gas other than the feed gas at a regeneration pressure (P″), such as: P″<P, at which the adsorbed compounds are desorbed, then recovered downstream of the adsorbent bed at this regeneration pressure (P″). On completion of this desorption step, the adsorbent can undergo a new adsorption step.        
The lower the regeneration pressure (P″), the more efficient the desorption of the undesirable compounds. This regeneration pressure (P″) therefore has a strong impact on the purity of the hydrogen produced, on the hydrogen recovery rate, and on the quantity of adsorbent needed.
In practice, the adsorbents generally used for treating H2/hydrocarbon mixtures require a regeneration pressure (P″) between 1.5 and 2 bar absolute, but always less than 3 bar absolute. This is because, if the regeneration pressure exceeds this maximum value, the species heavier than propane, which are always present in the gas stream to be purified, are permanently adsorbed on the adsorbent and rapidly poison it.
Moreover, at all petrochemical facilities, the hydrocarbon-rich waste gases from all the units, and in particular those from the hydrogen purification units, cannot be released to the atmosphere. They are collected on a fuel gas network which supplies the various burners of the facility.
The pressure of this network (P″′) is generally between 4 and 7 bar absolute, that is, always above 3 bar absolute in practice.
This explains why, due to the difference in pressure that exists, the waste gases from PSA units can never be sent directly to the fuel network of a petrochemical facility.
Many alternatives have already been proposed to try to solve this problem.
According to one known solution, the waste gas leaving the PSA unit is compressed by a rotating machine, such as a gas compressor, thereby raising the pressure of this waste gas from the pressure P″ to the pressure P″′ in order to introduce it subsequently into the fuel network of the petrochemical facility.
According to another known solution, the burners of several furnaces are replaced to permit the combustion of the gas at the pressure P″, which avoids having to compress it, as in the previous case.
However, these two known solutions are very costly because they generally increase the total cost of hydrogen purification by a factor of 1.5 to 3.
The problem which accordingly arises is to be able to purify the hydrogen-rich gases economically, in particular those containing at least one heavy hydrocarbon species of the C3+ type, that is, in which the number of carbon atoms is 3 or more, and without facing the problems encountered in the prior art.